This invention relates to a method for protecting the integrity of the reactor container base in nuclear power plants, and a device for implementing the method.
The proposed method and device are applicable to all reactor types (PWR or pressurized water reactor, BWR or boiling water reactor, HWR or heavy water reactor, etc.) and to all types of containing system (large dry type, steam suppression type, condensation on ice type, subatmospheric type etc.), although for simplicity the present text will describe only their application to a PWR with a steam suppression and large dry containing system.
U.S. Pat. No. 5,057,271 of the present applicant describes a system for protecting the base or foundation slab of the reactor containing building in nuclear power plants against erosion and degradation by molten material escaping from the reactor pressure vessel as as result of serious accident, the system comprising a chamber holding at least part of said pressure vessel and comprising below the vessel a cavity containing a structure immersed in a cooling fluid and composed of a plurality of superposed beams arranged suitably offset in layers, to intercept the molten material escaping from the reactor pressure vessel in the case of melt-down of its core, and to uniformly and three-dimensionally distribute the molten material over a large heat transfer surface, hence preventing its direct contact with the base of the cavity. According to the aforesaid patent each of said beams forming the structure or pile immersed in the cooling fluid can be of double T cross-section with a U-shaped upper flange and can be constructed for example of stainless steel. In this manner the molten material can be redistributed into a plurality of layers of small thickness to rapidly cool and resolidify, aided in this by the cooling fluid in which the pile of beams is immersed.
This very rapid contact between the molten material and the cooling fluid which floods the cavity below the reactor pressure vessel and in which said pile of beams is immersed gives rise however to the risk of so-called steam explosions due to the violent conversion of heat energy into mechanical energy. In order to avoid this risk, one embodiment of the aforesaid patent proposes to mount a suitable temporary collection structure below the reactor pressure vessel but above the level of the cooling fluid in said cavity, so that the molten material falls into the underlying fluid over an extended time.
The object of the present invention is to provide a solution to the problem of protecting the integrity of the reactor container base in nuclear power plants in which the potential risk of steam explosions is prevented more effectively, reliably and economically than in the aforesaid previous solution of the applicant.